1. Field of the Invention
This invention relates in general to a dynamic voltage threshold system for neutralizing intertrack cross-talk in multi-channel magnetic character recognition systems and more particularly to an improved dynamic or variable voltage threshold system responsive to an extremely wide dynamic range of information signals for accommodating an unusual increase or decrease in the magnetic flux generated by a portion of a character being read in the character recognition system.
2. Description of the Prior Art
In prior character recognition systems, cross-talk between adjacent data tracks or channels presented serious problems in differentiating between the document background and the encoded character. A system has been recently invented that dynamically controls the threshold voltage for neutralizing or rendering ineffective cross-talk between adjacent electromagnetic transducers in multi-channel magnetic character recognition systems. This system is fully described in U.S. Pat. No. 3,668,634 issued on June 6, 1972 of common inventorship and ownership herewith. This dynamic threshold voltage determination system discriminates between the document background and the magnetically encoded character quite effectively over a fairly wide dynamic range of information signals generated from adjacent electromagnetic transducing heads scanning adjacent portions of the magnetically encoded characters. This system has been commercially embodied in the B9134-1 reader sorter employing a MULTITRACK E 13B character recognition system manufactured by Burroughs Corporation, Detroit, Mich. In operation, this system properly and successfully recognizes magnetically encoded characters for over approximately 95% of the documents being read and sorted by the system. Thus, less than approximately 5% of processed documents are rejected because the magnetically encoded characters imprinted thereon cannot be recognized with a high degree of reliability.
The character recognition system employing the dynamic threshold voltage determination system of U.S. Pat. No. 3,668,634 handles a wide dynamic range of dynamic information signals. It has been determined that approximately 95% of the magnetically encoded documents processed through the prior dynamic threshold voltage determination system produces information signals in a 90% to 110% of nominal information signal range. Since over approximately 95% of processed documents were properly recognized, it can be appreciated that the prior dynamic threshold voltage determination system substantially reduced the document reject rate in character recognition systems, especially rejects attributable to undesirable cross-talk.
Any rejected document must, however, be processed individually by hand. Since this becomes a time consuming and expensive manual procedure for a large volume of documents, industry is continually making efforts to further reduce the number of documents rejected in character recognition systems.
Applicant has discovered that a major source of the problem of further reducing the number of documents rejected in a character recognition system employing a dynamic threshold voltage system is that some magnetically encoded characters produce an unusually small or large amount of magnetic flux, resulting in an information signal less than 25% or greater than 150% of a nominal information signal.
Applicant has also discovered that if a particular portion of a magnetically encoded character being scanned has been imprinted with an unusually excessive amount of iron oxide such that the resulting high strength magnetic flux produces an information signal having a voltage extreme or peak of over 300% of nominal. Applicant has further discovered that if the magnetically encoded character has been imprinted on a document distorted such that a portion of the magnetically encoded character is positioned extremely far from the electromagnetic transducing head, when the document is being read, then an information signal as low as 15% of nominal may also be produced. While either of these occurrences is unusual, they may cause the quantizers employed in a magnetic character recognition system to produce ambiguous pulses of non-uniform duration.